This invention is generally directed to layered photoresponsive devices, and more specifically, the present invention is directed to an improved layered photoresponsive device comprised generally of a transport layer situated between two photogenerating layers. The layered photoresponsive devices of the present invention are useful as imaging members in various electrostatographic imaging systems, including those systems wherein electrostatic latent images are formed on the imaging member. Additionally, the photoresponsive devices of the present invention can be initially charged positively or negatively, and development can be accomplished by employing the appropriately charged developer composition, thus, for example, when the device is charged negatively, a developer composition is employed wherein the toner material is charged positively.
Numerous photoresponsive devices for electrostatographic imaging systems are known including selenium, selenium alloys, such as arsenic selenium alloys; layered inorganic photoresponsive devices, and layered organic photoresponsive devices. Examples of layered organic photoresponsive devices include those containing a charge transporting layer and a charge generating layer. Thus, for example, an illustrative layered organic photoresponsive device can be comprised of a conductive substrate, overcoated with a charge generating layer, which in turn is overcoated with a transporting layer. In a further variation of this device, the charge transporting layer can be overcoated with the photogenerating layer. Examples of generating layers that can be employed in these devices include pigments such as selenium, cadmium sulfide, vanadyl phthalocyanine, x-metal free phthalocyanines, dispersed in binder resins, while examples of transport layers include dispersions of various diamines, reference, for example, U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference.
There nevertheless continues to be a need for improved photoresponsive devices, and improved imaging systems utilizing such devices. Additionally, there continues to be a need for photoresponsive devices of varying sensitivity, which devices are economical to prepare and retain their properties over extended periods of time. Furthermore there continues to be a need for photoresponsive devices that permit both normal and reverse copying of black and white images. An example of such an application resides in the production of a positive (black on white background) image copy from a negative (white on black background) original, such as a silver halide microfilm system wherein positive copies result from negative originals. This process can be accomplished by using two developers with toner compositions of opposite polarity as known in the art, however these systems are complex, and thus costly. It would thus be desirable to provide a system wherein there is utilized a single developer composition with a photoresponsive device that can be charged either positively or negatively. This requires an ambipolar photoresponsive device, which device contains materials that permit the photogeneration, and transport of both positive and negative charge that is, holes and electrons.